In the field of graphic arts, an image formation system providing ultrahigh contrast (especially .gamma. (gamma) exceeding 10) is required for achieving satisfactory reproduction of a continuous tone with a dot image or reproduction of a line image. An image formation system for obtaining ultra-high contrast photographic properties using a stable developing solution has been demanded. To meet this demand, an image formation system for obtaining a hard negative having a .gamma. exceeding 10 has been proposed, in which a surface latent image type silver halide photographic material containing a specific acylhydrazine compound is developed with a developer adjusted to pH 11.0 to 12.3 and containing 0.15 mol/l or more of a sulfite preservative, as disclosed in U.S. Pat. Nos. 4,166,742, 4,168,977, 4,221,857, 4,224,401, 4,243,739, 4,272,606, and 4,311,781. While conventional ultra-high contrast image formation systems were applicable only to a silver chlorobromide emulsion having a high silver chloride content, the new system has an advantage of applicability to a silver iodobromide emulsion and a silver chloroiodobromide emulsion. Furthermore, the developer used in this system permits addition of a high concentration of a sulfite preservative and therefore has improved preservation stability over lith developers which are allowed to contain only a little amount of a sulfite preservative. However, a developer at pH 11 or higher as used in the above system is instable due to the susceptibility to air oxidation and cannot withstand long-term preservation or use. Attempts have been made to obtain a high contrast image by developing a silver halide light-sensitive material containing a hydrazine compound with a developer having a lower Ph. For example, JP-A-1-179939 and JP-A-1-179940 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") disclose development processing of a light-sensitive material containing a nucleation development accelerator having a group which can be adsorbed on silver halide grains and a nucleating agent containing a similar group by using a developer having a pH of pH 11.0 or lower. However, the emulsion used is a silver bromide emulsion or a silver iodobromide emulsion, and the system is not sufficient in providing stable photographic properties against variation of development progress or variation of the composition of a processing solution.
U.S. Pat. Nos. 4,998,604, 4,994,365, and 4,975,354 disclose hydrazine compounds having an ethylene oxide repeating unit and hydrazine compounds having a pyridinium group. Judging from the description of Examples, these inventions do not provide sufficiently high contrast, and it is difficult to secure high contrast and a necessary maximum density under practical development processing conditions. Furthermore, light-sensitive materials having hard gradation enhanced through nucleation by a hydrazine derivative show large variation in photographic properties with pH change of a developer; the pH of a developer largely varies, for example, it rises due to air oxidation and thickening through evaporation of water and falls due to absorption of carbon dioxide in air. Therefore, various attempts to reduce the dependence of photographic properties on pH of a developer have been made.
On the other hand, light-sensitive materials for dot-to-dot work which are generally handled in a lighted room occupy a large proportion in the field of photomechanical processing. In this field, high reproducibility in formation of super-imposed letters even with a fine line width is demanded. To meet the demand, development of a nucleating agent having improved activity has been long awaited. The expectation for a nucleating agent with higher activity is particularly high for use in light-sensitive materials having such low sensitivity as to be handled in a lighted room because hard gradation enhancement by a nucleating agent hardly occurs in these materials.
In order to achieve these objects, highly active hydrazine nucleating agents have been developed as disclosed in JP-A-6-148828, JP-A-6-180477, and JP-A-6-194774.
In particular, nucleating agents having, as acyl group, an alkyl group substituted with at least one electron attracting group are excellent in that a very high contrast can be secured even with a developer at pH 11 or lower and that the variation of photographic properties due to exhaustion of a developer can be suppressed. Nevertheless, some of them are susceptible to oxidation and require further improvement in preservability.
Methods for obtaining a direct positive image which comprise surface developing an internal latent image type silver halide photographic emulsion in the presence of a nucleating agent and photographic emulsions or light-sensitive materials used therefor are known, e.g., from U.S. Pat. Nos. 2,456,953, 2,497,875, 2,497,876, 2,588,982, 2,592,250, 2,675,318, 3,227,552, and 3,317,322, British Patents 1,011,062, 1,151,363, 1,269,640, and 2,011,391, JP-B-43-29405 (the term "JP-B" as used herein means an "examined published Japanese patent application"), JP-B-49-38164, JP-A-53-16623, JP-A-53-137133, JP-A-54-37732, JP-A-54-40629, JP-A-54-74536, JP-A-54-74729, JP-A-55-52055, and JP-A-55-90940.
In the above-described methods for obtaining a direct positive image, the nucleating agent may be added to a developer but, it is more common to add it to a photographic emulsion layer or any other appropriate layers of a light-sensitive material.
Hydrazine compounds are the most well known of the nucleating agents that can be added to a direct positive silver halide light-sensitive material. Examples of the hydrazine compounds are described in Research Disclosure, No. 23510 (November, 1953), ibid, Vol. 151, No. 15162 (November, 1976), and ibid, Vol. 176, No. 17626 (December, 1978). In general, although hydrazine nucleating agents are the most excellent in discrimination, providing a large difference between a maximum density (Dmax) and a minimum density (Dmin), they are disadvantageous in that a high pH (pH 11 or higher) is required in processing, and improvement in this respect has been desired.
U.S. Pat. No. 5,252,426 discloses an arylhydrazide compound having a difluoroacetyl group or a monofluoroacetyl group as an acyl group. The U.S. patent also has a mention of compounds having directly bonded to the aryl group thereof a group which can be adsorbed to silver halide. However, any of the compounds disclosed does not exhibit sufficient performance for hard gradation enhancement by nucleation.
JP-A-3-125134 discloses a hydrazide compound having a difluoroacetyl group, but the compound disclosed turned out to fail to exhibit sufficient nucleating activity.
Highly active hydrazine nucleating agents disclosed in JP-A-63-234244, JP-A-63-234245, and the above-cited JP-A-6-148828, JP-A-6-180477, and JP-A-6-194774 are known as nucleating agents having a group accelerating adsorption to silver halide. However, many of them tend to be oxidized and require improvement in preservability. Thus, further enhancement of nucleating activity has been awaited for not only improved preservability but improved dot quality.